1. Field of the Invention
The present invention relates to a semiconductor display device using thin-film transistors. In particular, the invention relates to a semiconductor display device in which a pixel switching circuit and driver circuits are formed on the same substrate in an integral manner.
2. Description of the Related Art
In recent years, the techniques of forming semiconductor devices, such as thin-film transistors (TFTs), by using a semiconductor thin film formed on an inexpensive glass substrate have made rapid progress. This is because of increased demand for active matrix liquid crystal display devices.
In active matrix liquid crystal display devices, TFTs are provided for respective ones of hundreds of thousands to millions of pixel regions that are arranged in matrix and charge that enters or exits from each pixel electrode is controlled by the switching function of the associated TFT.
The basic configuration of an active matrix liquid crystal display device in which thin-film transistors are arranged will be described below with reference to FIGS. 34A and 34B. FIG. 34A is a sectional view obtained by cutting a liquid crystal display device by a plane perpendicular to a substrate, specifically taken along a chain line A-A′ in FIG. 34B.
An insulating film (not shown) is formed on the surface of a transparent base substrate 1. Reference numeral 2 denotes an active layer of a TFT; 3, a gate electrode; 4, a data line; 5, a drain electrode; 6, an interlayer insulating film; 7, a black matrix; 8, a transparent conductive film as a pixel electrode; and 9, an alignment film.
In this specification, the structure including the base substrate 1 and the other members mentioned above (including the TFTs) is called an “TFT substrate.” Although FIG. 34A focuses on a single pixel, actually the TFT substrate is composed of a pixel area including hundreds of thousands to millions of pixel switching TFTs (called pixel TFTs) and peripheral driver circuit areas including a number of TFTs for driving the pixel TFTs.
On the other hand, reference numerals 10-12 denote a transparent substrate, a transparent conductive film as an opposed electrode, and an alignment film, respectively. The structure including these members, which is opposed to the TFT substrate, is called an “opposed substrate.”
As shown in FIG. 35A, the TFT substrate 20 and the opposed substrate 30 are subjected to an alignment treatment such as rubbing for giving proper alignment to a liquid crystal. Thereafter, to control a substrate interval (cell gap) between the TFT substrate 20 and the opposed substrate 30, grainy spacers 41 are uniformly scattered over the entire surface of the TFT substrate 20. Then, a sealing agent 42 is printed. The sealing agent 42 has a role of an adhesive for bonding the substrates 20 and 30 together as well as a role of a sealing material for sealing the space between the substrates 20 and 30 to prevent a liquid crystal material that will be injected there from leaking to the outside of the substrates.
FIG. 36 is a sectional view of the TFT substrate 20. Since the grainy spacers 41 are uniformly scattered over the entire surface of the TFT substrate 20 to control the cell gap, the spacers 41 exist in not only the pixel area 22 but also the peripheral driver circuit regions 23 as shown in FIG. 36. Usually, the pixel TFTs formed in the pixel area 22 are not much different in device size from the driver circuit TFTs formed in the driver circuit areas 23. However, the black matrix for covering the pixel TFTs, the pixel electrodes that are transparent conductive films, and other members are formed in the pixel area 22. Further, in reflection-type liquid crystal display devices, a reflective electrode is formed in the pixel area 22. On the other hand, connection lines necessary to constitute CMOS circuits for driving the pixel TFTs are formed in the driver circuit areas 23. Therefore, there are differences in the height (distance) from the surface of the base substrate 1 between the pixel area 22 and the driver circuit areas 23.
A description will now be made of a case where the height as measured from the surface of the substrate 1 in the pixel area 11 is greater than in the driver circuit areas 23. The grainy spacers 41 are scattered in not only the pixel area 22 but also the driver circuit areas 23 by a wet or dry method. If the grainy spacers 41 have approximately uniform sizes, they have differences in the height as measured from the substrate 1 depending on their positions. Now, the height of the top of each spacer 41 in the pixel area 22 and that of the top of each spacer 41 in the driver circuit areas 23 are represented by hp and hd, respectively. As seen from FIG. 36, a height difference Δh=hp−hd occurs due to the difference in height between the pixel area 22 and the driver circuit areas 23.
Then, as shown in FIG. 37A, the TFT substrate 20 and the opposed substrate 30 are bonded together with the sealing agent 42. Thereafter, the space between the TFT substrate 20 and the opposed substrate 30 are filled with a liquid crystal material 43 and a liquid crystal injection inlet 44 is sealed with a sealing material (see FIG. 37B). In this manner, an active matrix liquid crystal display device having the configuration shown in FIG. 34A is obtained.
However, the liquid crystal display device having the above configuration has the following problems.
Because of the height difference Δh that is caused by the difference in height between the pixel area 22 and the driver circuit areas 23, the cell gas cannot be made uniform, that is, a cell thickness variation occurs, when the TFT substrate 20 and the opposed substrate 30 are bonded together. Further, as shown in FIGS. 37A and 37B, strain occurs in the opposed substrate 30. Defects such as display unevenness and an interference fringe (on the top surface of the opposed substrate) may occur in a liquid crystal display device having a cell thickness variation and strain in the opposed substrate 30.
Where the height as measured from the substrate 1 in the driver circuit areas 23 is greater than in the pixel area 22, because of the above-described height difference Δh, unduly strong force is exerted on the spacers 41 that are scattered in the driver circuit areas 23 when the TFT substrate 20 and the opposed substrate 30 are bonded together. As a result, the driver circuit TFTs having a more complex structure than the pixel TFTs are damaged considerably, which adversely affects the yield of products.
Where grainy spacers 15 exist in the pixel area, disorder in image display (disclination) may be observed as shown in FIG. 34B because the alignment of the liquid crystal material is disordered in the vicinity of the spacers 15.
As described above, where the cell gap is controlled by using conventional grainy spacers, satisfactory display may not be obtained due to various factors.
In liquid crystal display devices that are commonly manufactured or manufactured as trial products, the cell gap appears to be set at 4-6 μm irrespective of the pixel pitch. However, in the future, liquid crystal panels will be required to have higher resolution and hence the pixel pitch will be increasingly reduced.
For example, projection-type liquid crystal display devices are desired to be able to display images having as high resolution as possible in view of the fact that the images are projected onto a screen in an enlarged manner. Also from the viewpoint of the cost, the optical system needs to be miniaturized and the panel size needs to be reduced. For the above reasons, in the future, it will be necessary to manufacture liquid crystal display devices having a pixel pitch of 40 μm or less, preferably 30 μm or less.
In liquid crystal display devices for displaying such high resolution images, even grainy spacers of several micrometers in diameter may deteriorate display quality when they exist in the effective display area.
Further, when a liquid crystal material is injected, the flow of the liquid crystal material forces conventional grainy spacers themselves to flow. As a result, a uniform spacer dispersion density profile may not be obtained, to cause a cell thickness variation.
Because of their characteristics, liquid crystal display devices using a ferroelectric liquid crystal that attract much attention recently and reflection-type liquid crystal display devices are required to have small cell gaps.
However, with conventional grainy spacers, it is generally difficult to produce a cell having a small, uniform-profile cell gap.